Flashlamp units employing solid state switching devices to fire a plurality of flashlamps individually and in sequence are well-known. In U.S. Pat. No. 3,458,270 there is disclosed such a switching arrangement having switch devices of silver oxide adhesively bonded to the substrate of an electrical circuit with an organic polymer binder. The particular flashlamps employed are further depicted to be of the low voltage type which upon battery ignition convert the switch devices in the electrical circuit to a "low ohmic" condition. The organic binder in the switch material composition is said to produce a chain of reduction and oxidation reactions with the silver oxide which occurs simultaneously and exothermally in converting to a metallic silver state.
The conversion of silver oxide to metallic silver as a switching device in a photoflash unit is also well-known for use in the same manner with high voltage type flashlamps. For example, in a more recently issued U.S. Pat. No. 3,990,833, assigned to the assignee of the present invention, there is disclosed a switch material composition having silver oxide mixed with a carbon containing silver salt and a humidity resistant organic polymer binder which is converted by the radiant energy provided when a joining flashlamp is fired to metallic silver having a relatively low electrical resistance. The use of silver oxide as the sole silver source in the switching material composition is said to be objectionable for said type photoflash unit as being overly sensitive and proves to react too vigorously during conversion. That is, the silver oxide converts too vigorously and damages the circuitboard or the electrical circuit. Since the continuity of the electrical circuit to the next unfired flashlamp is also likely to be destroyed by such action, a carbon containing silver salt is mixed in the switch material composition to reduce this overall problem. In so doing, the electrical resistance of the converted switch is said to be in the range 0.1-10 ohms.
In the above referenced Ser. No. 451,505 application there is described a radiant energy responsive switch composition adapted for use in a low voltage firing circuit by exhibiting an electrical resistance not exceeding about 0.3 ohms after conversion. Said switch composition employs in weight percent approximately 51-72% silver oxide, approximately 20-36% powdered metal exhibiting thermal and electrical conductivity which does not react exothermally with silver oxide and approximately 8-13% organic polymer binder. In the preferred material composition, a composite organic polymer binder is employed wherein 3-6 weight percent of a first organic polymer binder, such as ethyl hydroxyethyl cellulose, serves as a fuel undergoing combustion for chemical reduction of the silver oxide component while a second organic polymer, such as black phenolic resin, makes up the remaining 5-7 weight percent to maintain continued adhesion of the converted switch to the circuitboard member. The preferred powdered metal in said switch composition is copper metal which is said to retain sufficient electrical resistance in the switching device before conversion to the electrically conductive state. The preferred method of applying said switch composition to the circuitboard is said to be silk screening from an organic liquid suspension whereby the material is deposited across a pair of spaced apart terminals in the electrical circuit.
Certain reliability problems have been experienced with the above described preferred switch composition due to some lack of operational sensitivity and storage stability of the liquid suspension during product manufacture. Only partial switch conversions have been experienced as the result of variable amounts of radiation being emitted from the associated flashlamps and which has further limited the product design by requiring close physical proximity of each switch to the actuating flashlamp. Moreover, a partially converted switch can produce higher circuit resistance leading to out of sequence flashing or failure of the flashlamps to flash. While increasing the proportion of copper metal powder in the switch composition was found to increase sensitivity of the switching device to a more complete conversion, such change also undesirably lowered the electrical resistance in the unconverted device below the minimum 1,000 volt level. Storage stability of the liquid switch suspension was found to be caused by partial solubility of the phenolic resin component in the organic solvent which produced viscosity variation leading to non-uniform switching devices being deposited therefrom as well as further loss in conversion sensitivity by coating the silver oxide particulates with a barrier layer.
A need still remains therefore for these switches to exhibit higher sensitivity in operation along with greater reliability in manufacture. Any modification of the switch material composition to accomplish these results must further remain stable under conditions of elevated temperature and high humidity by reason of experiencing such conditions during storage and use.